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      平滑加权轮询算法 1. 算法原理 nginx的平滑轮询负载均衡采用了类似的算法：
 最开始按初始权重(a1,a2,&hellip;,an)分配，权重序列记作b1,b2,&hellip;,bn； 挑选当前权重序列中最大的作为被选中项，记作bi； bi=bi+ai-初始权重和，除bi外其余项bk=bk+ak。 重复第2和3步进行轮询  算法示例如下：（*号代表当前被选中的记录）
1 2 3 4 5 6 7 8 9  初始权重：{40,40,20} { 40*, 40 , 20 } + {-60,+40,+20} {-20 , 80*, 40 } + {+40,-60,+20} { 20 , 20 , 60*} + {+40,+40,-80} { 60*, 60 ,-20 } + {-60,+40,+20} { 0 ,100*, 0 } + {+40,-60,+20} { 40*, 40 , 20 } + {+40,+40,+20} ...   可以看到，循环权重序列最大公约数和的次数时，权重值会回到初始序列。上例中是2:2:1，即5次。
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       计算机组成原理及体系结构中的那些算法 操作系统中的那些算法 计算机网络中的那些算法 
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      计算机硬件由三部分组成：CPU、存储器、I/O设备。这里提到的存储器指的实际上是主存（cache和register一般在CPU中）。磁盘、光盘、磁带等低速存储设备则常被用于存放程序的输入或者输出数据。
闪存读取时间&lt;100ns，写入需要4-10us。
典型的寻道时间2-30ms，平均寻道时间是寻道时间的平均值，一般在4-10ms之间。
平均旋转等待时间是磁盘旋转一周时间的1/2。例如5400rpm（每秒90转），转动一周时间约为11.11ms，其平均旋转等待时间为5.56ms。
访问时间=寻道时间+旋转等待时间，一般在8-20ms之间。
磁盘数据传输率是从磁盘获得数据或者向磁盘存储数据的速率，25-100MB/s。对于磁盘的内侧磁道，数据传输率低于外侧磁道。
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      我们希望拥有一个私有的、无限大容量的、速度无限快的，断电不会丢失数据，并且价格低廉的存储器——但很遗憾，目前没有这样的存储设备。存储设备有很多种类，不同的存储设备各有特点。
计算机硬件由三部分组成：CPU、存储器、I/O设备。这里提到的存储器指的实际上是主存（cache和register一般在CPU中）。磁盘、光盘、磁带等低速存储设备则常被用于存放程序的输入或者输出数据。
1. 存储分层体系（按速度和价格分） 目前计算机系统使用的存储体系是一种分层存储器体系。在该体系中，由上到下，速度越来越慢，价格越来越低。在主存（RAM）以下的都可以认为（与RAM比）是低速存储设备。
主机内部的存储器称为主存(主存)，一般为RAM，主机外部的存储器称为辅存(外存)，一般为磁盘、磁带、光盘、闪存（ROM的一种）。注意，寄存器和cache是比较特殊的存储器，通常是SRAM，寄存器通常用于CPU内部暂存数据使用，常见的cache位于CPU内，用于解决CPU和主存速度不匹配的问题(主存存取速度比CPU运算速度慢很多)。
从寄存器，到cache，到主存，再到闪存、磁盘、磁带/光盘，价格越来越低，速度越来越慢，但在计算机中的容量通常越来越大。
 寄存器(register)：由硬件管理，register同主存之间的数据交换对所有程序员都是透明的 cache：由硬件管理，cache同主存之间的数据交换对所有程序员都是透明的 主存：主存目前一般都是RAM。主存的管理由硬件和操作系统来完成，即主存同磁盘、光盘、磁带等低速设备之间的数据交换对应用程序员是透明的 ROM：同磁盘，低速非易失性存储器被抽象称为文件进行管理，文件模型对应用程序员来说是可见的 磁盘：磁盘等低速非易失性存储器的管理则被抽象称为文件进行管理，文件模型对应用程序员来说是可见的 光盘、磁带：同磁盘，低速非易失性存储器被抽象称为文件进行管理，文件模型对应用程序员来说是可见的  2. 按存储介质分  半导体存储器：RAM(SRAM、DRAM)、ROM(MROM、PROM、EPROM、EEPROM、FLASH)。半导体存储器都是随机访问设备，易失性存取，但注意ROM是非易失性的。注意，RAM即随机存储器是一个名词误用，事实上半导体存储器都是随机存取的。 磁介质存储器：磁表面存储器(磁盘、磁带)、磁芯存储器(已淘汰)。磁介质存储器都是串行访问设备，非易失性存取。串行访问设备可分为顺序访问设备(如磁带、光盘)和直接访问设备(即部分串行，如磁盘)。 光介质存储器：光盘。光介质存储器也是串行访问设备，非易失性存取。  3. 按存储易失性分 存储设备具有易失性是指设备断电后讲丢失所有数据内容。
如前所述，半导体存储器中RAM属于易失性存储。半导体存储器中的ROM，以及磁介质存储器和光介质存储器都属于非易失性存储。
4. 按存取方式分 半导体存储器属于随机存取设备，磁介质和光介质存储器都属于串行存取设备。
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       1. SRAM 典型的为6管1位NMOS静态RAM单元。使用触发器存储信息，相比DRAM有速度快的优点，但集成度低且成本高。
2 DRAM DRAM的刷新对CPU是透明的，即不依赖外部访问；刷新单位是行，故操作时仅需要行地址；刷新操作类似于读操作(刷新一次时间约为一个存取周期)，但又有所不同，刷新仅是给栅极电容充电，不需要信息输出，另外刷新时不需要选片，即整个存储器中的所有芯片同时被刷新。
典型的为1管NMOS动态RAM单元。使用电容存储信息，与SRAM有以下不同：
 ①属于破坏性读出，需要读后重写。 ②由于电容漏电，需要定时刷新(通常2ms刷新一次，方式有集中式、分散式、异步式)。 ③采用地址复用技术，地址线是原来的1/2，地址信号分行列两次传送(RAS和CAS控制，低电平有效)。  
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